BALB/c mice that were specially bred to carry and express only immunoglobulin (Ig) genes of the C57BL allotype (Ig Ch Gb) constitute the primary biologic material for this research proposal. A different number of backcrosses preceded the derivation of several such congenic mouse strains (CB-17, CB-23 and CB-26). This means that the chromosome (CB) containing the Ig CH Gb marker was paired repeatedly with its :ALB/c homologue for a different number of generations prior to deriving mice that were homozygous for the CB chromosome. As a result of genetic recombinations, we would expect the CB chromosome to acquire many BALB/c genes including some BALB/c Ig genes. The hypothesis that we will test is as follows: (1) that continued backcrossing results in a diminishing number of C57BL Ig genes on the CB chromsome partly at the expense of an increasing number of nontranscribed, BALB/c recombinant Ig genes; (2) that the quiescence of the latter is due to the absence of specific regulator genes that may be under thymus control; and (3) that in the breeding of CB mouse strains, one selects against the acquisition of BALB/c regulator genes. We will try to reveal such gene differences by means of testing the immunocompetence of CB mice to different antigenic determinants as a function of age. We will breed selectively those CB mice that may be distinguished in this way to see whether we can derive congenic mouse lines that cannot synthesize certain antibody specificities. Also, we will try to substitute for putative immunologic deficiencies by reconstituting thymectomized, X-irradiated CB mice with thymocytes of BALB/c mice that carry and express only the Al/N allotype (Ig CH Gc). These experiments will benefit from our application of a solid-phase radioimmune assay that can measure quantitative and qualitative changes in Ig's.